This invention relates to a centrifuge and to an associated method of operating a centrifuge. The apparatus and method of the invention are particularly, but not exclusively, applicable in cantilever screen-scroll type centrifuges.
Conical screen-scroll centrifuges have been used to dewater thickened slurries from nominally 40-60% feed solids to nominally 80-95+% solids (or 20-5% cake moisture). As illustrated in Fig. 1A, such centrifuges comprise a scroll conveyor 10 surrounded by a screen basket 12 and disposed therewith in a housing 14. Scroll conveyor 10 and screen basket 12 are cantilevered from a support 16 at one end. At that same end, conveyor 10 and screen basket 12 are operatively connected to a single input, dual output planetary gear box or a cyclo gear box 18 which is driven by a motor 20. A feed pipe 22 extends into an open, free end of scroll conveyor 10 for delivering a thickened feed slurry thereto. The feed slurry exits an opening (not shown) in a hub 24 of conveyor 10 and is deposited onto screen basket 12. Solids 26 in the slurry are conveyed along an inner surface of screen basket 12 to a conical discharge 28 by a helical blade 30 of conveyor 12, while filtrate is discharged at 32 through screen basket 12.
A simple cross-sectional schematic of the screen-scroll centrifuge of FIG. 1A is shown in FIG. 1B. Feed slurry introduced via feed pipe 22 into a feed cone 34 of conveyor 10 is accelerated in the feed cone (arrows 36) so that when the slurry is laid onto a small diameter end 38 of screen basket 12, the slurry has acquired the proper G-force to effect filtration of the bulk liquid followed by dewatering (arrows 32) so that the remaining liquid trapped in the cake pores can be further released with time. The dewatering process is facilitated by continuously thinner cake and an increasing higher centrifugal force as the cake moves toward discharge at a larger screen diameter 42. Washing can be applied to remove the impurities in food, chemical, and mineral applications, wash liquid being introduced at small diameter 38 of conical screen basket 12 shortly after the feed zone. The washed cake is ultimately dewatered at the larger screen diameter 42. The screen drain filtrate (arrows 32) and the cake (arrow 44) are collected respectively in separate hoppers (not shown) for downstream processing.
One key benefit of the cantilever screen scroll design as illustrated in FIGS. 1A and 1B, is that both scroll conveyor 10 and screen basket 12 are opened at the front end of the machine. This allows the operator easy access to the rotating assembly for regular maintenance such as replacement of worn components (e.g. screen, worn and broken tiles, scroll, nuts and bolts), and removal of foreign objects trapped in the process streams, as well as regular visual inspection of the process during operation to assure satisfactory operation. Because the screen scroll centrifuge is a cantilever design, another advantage is that only a set of supporting bearings located at one end of the machine is required instead of two bearings associated with a horizontal end-to-end support. This minimizes significantly the overall cost of the machine. However, there is a disadvantage in that the overhung moment from the pivot or support may limit the cantilever mass as well as the distance of cantilever mass from the pivoted bearing or support. This may also result in a rotational speed limitation owing to natural frequency considerations. Another limitation of the screen-scroll-type centrifuge is that the feed has to be pre-thickened to nominally 40-60% before introduction to the screen to remove a majority of the bulk liquid. This thickening can be achieved, for example, with hydrocyclones, thickening tanks or thickening screens upstream of the dewatering screen scroll.
In a different approach, both thickening and dewatering are combined in a single unit using a screen bowl centrifuge as shown in FIG. 2. A solid-bowl configuration comprises a cylindrical bowl 46 followed by a conical beach 48 used for separation and thickening of the separated solids to form a cake. A cylindrical screen 50 downstream of the conical beach is used to further dewater the cake to lower the moisture content thereof. Consequently, dilute feed with solids content by weight of 5-50% can be used. This is advantageous over the screen scroll where only thickened feed of nominally 40+% is permissible.
The prior art centrifuge of FIG. 2 also includes a worm-type conveyor 52 for scrolling cakes solids along inner surfaces of bowl 46, beach 48, and screen 50. Effluents are discharged from a clarifier pool 54 into a centrate discharge chamber or hopper 56 of a centrifuge casing 58. Filtrate is discharged through screen 50 into a filtrate drainage chamber or hopper 60 of casing 58, while cake 62 is discharged into a solids discharge chamber or hopper 64. A feed slurry is fed into a hub 66 of conveyor 52 via a feed pipe 68. Conveyor 52 and bowl 46 are rotatably supported at opposite ends on bearings 70 and 72 and are differentially rotated via a gear unit 74.
In another variation of the screen-bowl-type centrifuge, shown is FIG. 3, a cylindrical screen section 76 is provided at a larger diameter than the diameters of a cylindrical solid bowl section 78 and a bowl section 80. A first helical conveyor blade 82 conveys cake solids along inner surfaces of bowl section 78 and bowl section 80, while a second helical conveyor blade 84 conveys cake solids along an inner surface of screen section 76. Conveyor blades 82 and 84 are rigid with a conveyor hub 86 and accordingly rotate at the same angular velocity which is slightly different from an angular velocity of screen section 76, bowl section 78 and bowl section 80.
An advantage of the design of FIG. 3 is that cake dewatering on screen section 76 is carried out at a higher G-force. A disadvantage is that as the feed as laid abruptly onto screen 76, the feed is underaccelerated, i.e., the tangential speed of the feed is much less than that of screen 76 at a solid-body rotation. This difference in tangential speed results in slippage of the feed on the screen surface as the feed is being accelerated by the screen surface, thereby causing high wear on screen 76 especially for abrasive feed materials. Furthermore, it can be shown that the undesirable radial velocity of the feed stream increases at the expense of a lower tangential speed (conservation of angular momentum). This in turn results in an increased solids penetration through screen 76, with a lower solids recovery or capture. The feed particle size can be further reduced through slippage of feed on the screen with the consequence of particle attrition which results in more loss of these fine solids through the screen. In all cases of this variation of the screen-bowl-type centrifuge, the screen bowl is horizontally arranged and supported by two bearings 88 (only one shown) at the two ends. The cost of this design is somewhat greater than the cantilever screen scroll design (FIGS. 1A and 1B) and the operator cannot access the rotating assembly as readily as in a cantilever screen scroll design.
An improvement in that direction is a cantilever screen bowl design as shown in FIG. 4. The unit includes a cylindrical bowl 90 and a conveyor 92 both rotatably cantilevered from a support located at the large diameter side of the machine. Because of this arrangement, in order to reduce the overhung bending moment, the length of the solid-bowl section 90 as well as the length of a cylindrical screen section 94 must be trimmed. The rotational speed of the machine may also limited owing to natural frequency considerations. These factors render the overhung shorter screen bowl design less effective with major disadvantageous results of lower throughput, wetter cake and dirtier effluent as compared to a regular screen bowl supported by two end-to-end bearings with the same diameter.